268 THE LAMELLIBRANCHIA 
stage (Fig. 242). Genera—Unio, Retzius ; shell thick, the hinge toothed. 
This genus includes more than a thousand species, the majority from the 
northern hemisphere. Anodonta, Lamarck; shell thin; the hinge without 
teeth ; British. Psewdodon, Gould. Quadrula, Rafinesque. Arconaia, Conrad. 
Monocondylaea, VOrbigny.  Solenaia, Conrad. Mycetopus, d’Orbigny ; 
foot cylindrical, with a terminal swelling ; South America, Famity 25. 
Mvure.rpAk, Gray. This family differs from the Unionidae in having two 
pallial sutures and a distinct branchial orifice; the shell is never fur- 
nished with lateral hinge teeth. Freshwater. Genera—Mutela, Scopoli. 
Pliodon, Conrad. Spatha, Lea. Jridina, Lamarck. Hyria, Lamarck. 
Castalia, Lamarck.  Aplodon, Spix. Plagiodon, Spix. Famity 26. 
AETHERIDAE, Adams. Shell irregular, generally fixed in the adult state. 
Mantle with a single suture; foot absent; anterior adductor muscle 
Fic. 241. 
Modiolaria trapezina, Lamarck, viewed from the left side ; the left mantle lobe is removed. 
ad’, ad’, anterior and posterior adductor muscles; br’, br’, internal and external gill-plates ; 
gl.p, foot gland ; 0.a, 0.b, anal and branchial orifices of the mantle ; 0.by, byssal orifice of the 
foot ; 0.p, pedal orifice of the mantle ; p, foot; pal, labial palp ; re.p, posterior retractor muscle 
of the foot. 
sometimes reduced or absent; from fresh water. Genera— Aetheria, 
Lamarck ; anterior adductor well developed ; African. Miilleria, Férussac ; 
no anterior adductor; American. artlettia, Adams. 
Susp-ORDER 3. TELLINACEA. 
Eulamellibranchia in which the mantle is not extensively closed, 
with two pallial sutures and two well-developed siphons; the gills 
smooth. The foot is compressed and elongated. The labial palps very 
large. Dimyarian; the pallial line has a deep sinus. 
Famity 1. Trnnirpar, Deshayes. The external branchial plate 
directed upwards (Fig. 206, H). The siphons separate and elongated. 
Foot with a byssogenous apparatus. Palps very large. Ligament of shell 
external. Genera—VTellina, Linnaeus; slightly inequivalve ; foot large ; 
British (Fig. 190). Gastrana, Schumacher ; equivalve ; the foot slightly 
developed ; British. Capsa, Bruguitre. Macoma, Leach, Faminy 2. 
ScROBICULARIIDAE, Adams. External gill-plate directed upwards. 
Siphons separate and excessively long. Foot without a byssus. The 
ligament partly internal, lodged in a concavity in the hinge. Genera— 
